1. Field of Invention
The present invention relates to communication systems. In particular, the present invention relates to demodulators which use a phase tracking loop (PTL) to track the phase of a modulated signal waveform.
2. Discussion of the Related Art
In burst communication systems, particularly digital communication systems comprising a communication transmitter for digital data transmission and a communication receiver for digital data reception via a channel, it is known to impress intelligent information to be conveyed onto a carrier for transmission by one of many different modulation techniques, including binary phase shift keying (BPSK) modulation or quaternary phase shift keying (QPSK) modulation. The burst consists of a preamble portion and a data portion. A demodulator in the communications receiver includes a phase tracking loop (PTL) which determines an initial estimate of the phase of the modulated signal using the preamble portion. The phase tracking loop is initialized with the phase estimate and thereafter constantly calculates an estimate of the transmitter""s phase so that it continuously tracks the incoming signal during reception and demodulation of the data portion.
Conventionally, a demodulator uses a phase tracking loop to track and coherently demodulate the modulated signal waveform received from a transmitter so that it may be transformed back into the fixed phase space of the transmitter. There are a number of different types of phase tracking loops employing phase locked principles such as squaring loops, Costas tracking loops, and decision-directed feedback loops for performing phase tracking of either a BPSK or QPSK modulated signal. A commonly used method for performing this type of phase tracking is a digital decision directed phase locked loop (DD-PLL). The basic principle of decision directed phase locked loops (DD-PLLs) is well known as described in the classic xe2x80x9cTelecommunication Systems Engineeringxe2x80x9d text by William C. Lindsey and Marvin K. Simon, originally published by Prentice-Hall in 1973, and the xe2x80x9cDigital Communicationsxe2x80x9d text by Kamilo Feher, originally published by Prentice-Hall in 1983 and republished by Noble Publishing Corp. in 1997. Generally, the input to a digital decision directed phase locked loop (DD-PLL) typically consists of only the phase angles of a sequence of complex data sample pairs obtained by down converting the incoming BPSK or QPSK modulated signal to a baseband quadrature (orthogonal) pair, 10 digit combination, passing these through matched filters and sampling the results at the symbol rate. This sampled pair may be considered as a complex variable in rectangular form. The complex variable is converted to polar form to produce the equivalent variable pair. The apparent incoming phase is referenced to the currently estimated phase (i.e. the tracked phase) to form the phase difference. The phase difference between the incoming phase and the estimated phase is influenced by the true difference between the phase systems of the transmitter and the receiver, by phase and thermal noise present at the receiver, and also by the symbol""s data content which changes the angle by a multiple of xcfx80/2 for QPSK or of xcfx80 for BPSK. The polar form is then transformed back into the rectangular form, for subsequent processing, including soft decision decoding when error control is being utilized.
In conventional phase tracking circuits, the effect of the data content on the phase difference between the incoming phase and the estimated phase is compensated by making a xe2x80x9chardxe2x80x9d decision on the data content of each individual BPSK or QPSK symbol on the rectangular coordinates. A standard phase detector generates phase error measurements for each BPSK or QPSK symbol, based on the hard decision of each symbol. In the absence of noise in the baseband quadrature pair, the estimated phase decision, which is based on each individual BPSK or QPSK symbol, is always correct so that the resultant phase error measurement equals the true difference between the phase systems of the transmitter and the receiver. The value of the resultant phase error measurement is then filtered to yield an updated estimate for use at the next symbol epoch, forming a classical servo loop.
In practice, noise is always present so that the resultant phase error measurement may be grossly distorted, especially when an incorrect decision is made in converting the phase difference between the incoming phase and the estimated phase to the resultant phase error measurement. As long as the bit error rate (BER) is small, many existing symbol-by-symbol decision directed phase locked loops (DD-PLLs) perform well. However, at low signal-to-noise ratios, the BER can be relatively high which means that the phase detector can also be unreliable. The initial phase error can be as much as +/xe2x88x9230 degrees when phase tracking of the data portion of the burst begins. Although correction algorithms such as Reed-Muller can be used to correct random errors, high initial phase error nevertheless results in high codeword error rates because it is difficult for the phase locked loop to lock and to correct for large phase errors. The effect of large initial phase errors, together with the large amount of noise entering the loop, may cause the demodulator to perform unacceptably when demodulating the beginning of the data portion. Indeed, the presence of large phase errors, either initially or during tracking, typically results in dropped cells. For burst communication systems, such as time division multiplexed access (TDMA), and especially for satellite communication systems with low signal-to-noise ratios, there is a need to reliably demodulate and decode the data portion of each burst and to reduce the number of dropped cells. Eliminating large errors in the initial phase estimates in the demodulator phase tracking loop can reduce the number of dropped cells.
For at least the above reasons, conventional decision directed phase locked loops (DD-PLLs) may fail to adequately track the phase of a phase shift keying (PSK) modulated signal, and to minimize the error rate for recovered data, especially significant errors at the beginning of phase tracking which result in dropped cells. This consequence is particularly damaging for digital communication systems such as satellite communication systems that utilize error correction codes and large constellation signal sets to communicate at very low signal-to-noise ratios. FIG. 8 graphically illustrates the relationship between the actual phase of a communications waveform and the estimate of the communications waveform in a phase tracking loop of a demodulator over time.
Accordingly, it is an object of the present invention to provide a communications receiver for use in satellite communication systems which compensates for possibly large errors in the initial phase estimates of a phase tracking loop and reduces the codeword error rate at the beginning of phase tracking due to poor initial phase estimates.
It is further an object of the present invention to provide a communications receiver using a short block of biorthogonal codes (e.g., Reed-Muller codes) within a phase tracking loop (PTL) of a demodulator and to improve the cycle slip and cell loss rate (CLR) when tracking the phase of a phase shift keying modulated signal waveform.
Yet another object of the invention is to run a group of data symbols at the beginning of a communications waveform backwards through the phase tracking loop in a demodulator to correctly demodulate that first group of data symbols.
These and other objects of the present invention may be achieved by a demodulator for demodulating a modulated signal waveform in a data communication system, comprising: a phase tracking loop tracking the phase of said modulated signal waveform and having an inner block decoder configured to decode a set of vector pairs of the modulated signal waveform at a decode rate to generate associated codewords and phase estimates, wherein a group of data symbols consisting of the first data symbols of the modulated signal waveform are run backwards through the phase tracking loop; and an outer block decoder and which utilizes and corrects only codewords associated with symbols after the group of data symbols consisting of the first data symbols of the modulated signal waveform. The outer block decoder receives all of the groups of data symbols, but there will probably be fewer errors in the first group of data symbols than there would have been if the data was not run backwards.
In accordance with yet another aspect of the present invention, a communication receiver uses a demodulator demodulating a modulated signal waveform from a transmission channel which is encoded by a sequence of codewords, comprising: a down converter which generates a succession of baseband signal samples of said input modulated signal including an in-phase component and a quadrature-phase component; a first converter which converts said succession of baseband signal samples of said input modulated signal from a rectangular form into a pair of polar coordinates having an incoming phase; a phase tracking loop which estimates the phase of said input modulated signal, said phase locked loop comprising: a comparator which generates a phase difference of said incoming phase of said input modulated signal and an estimated phase; a second converter which converts said polar coordinates having said phase difference into a set of vector pairs of phase stabilized observables in said rectangular form; a first block decoder which decodes said set of vector pairs of phase stabilized observables in said rectangular form at a decode rate to generate associated codewords; and a loop filter which filters said phase error estimate from said block decoder to yield an update of said estimated phase at each codeword, wherein a group of data symbols consisting of the first data symbols of the modulated signal waveform are run backwards through the phase tracking loop and a second block decoder which receives the associated codewords generated by said inner block decoder and which utilizes and corrects only codewords associated with symbols after and including the group of data symbols consisting of the first data symbols of modulated signal waveform that have been run backwards through the phase tracking loop.
The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example.